Remove unsafe from Window::from_ref and make Window clonable.

[iddm]

I think this "unsafe" is useless here, and the method is actually safe:

    #[inline]
    /// Create a new `Window` without taking ownership of the `WindowContext`
    pub const unsafe fn from_ref(context: Rc<WindowContext>) -> Window {
        Window { context }
    }

Having it is unsafe:

1. Is not useful.
2. It may confuse people who don't read. Go deeper to read the code.
3. It may lead to people seeing it preferring not to store the Rc<WindowContext> but to have a double indirection and reference counting: Rc<Window> which internally has Rc<WindowContext> for no reason but to avoid unsafe calls.

Another thing to improve is:

pub struct Window {
    context: Rc<WindowContext>
}

This struct can easily derive Clone due to having Rc inside; why doesn't it do so? Now, if we want to have the rich interface of Window, we must either keep it reference counted (Rc<Window>) or store the Rc<WindowContext> and create the Window object back from it when we need to use it: Window::from_ref(window_context). I think this experience is suboptimal and doesn't bring any benefit.

If it is agreed to change in favour, there is a PR: #1368

[iddm]

Closed due to the PR having been merged.